Many types of transmission elements are in use in robot design. The purpose of the transmission is to transmit mechanical power from a source to a load. Choice of transmission elements depends on power requirements, the nature of the desired motion, and the placement of the power source with respect to the joint. The primary considerations in transmission design are stiffness, efficiency, and cost.
Gears are the most common transmission elements in robots today. Factors to consider in gear design are material choice, choice of material surface treatment, and manufacturing precision. Considerations in designing geared transmissions are gear ratio, type of gear, gear shaft support, control of center distances, and lubrication.
Spur gears are most commonly used for parallel axis transmission or prismatic motions in rack and pinion configuration. Spur gears have the advantage of producing minimum axial forces, which minimize the need for controlling play in the gear mount.
A number of U.S. patents disclose geared transmission elements in wrist mechanisms. For example, the Niitu et al U.S. Pat. No. 3,784,031 discloses an early example of geared transmission elements for use in a wrist mechanism.
The Richter U.S. Pat. No. 4,030,617 discloses a wrist mechanism having concentric drive shafts and spur and bevel gear trains. However, the design is relatively expensive due to the large number of components. The Kimura et al U.S. Pat. Nos. 4,574,655 and 4,594,918 disclose the use of bevel gears and speed reduction devices in their respective three-axes wrist mechanisms.
Helical gears are also used in robot transmissions. They have several specific advantages. Because gear reductions are often quite large in robot transmissions, lack of adequate gear tooth contact ratio can be a problem. For given gear ratios and gear sizes, helical gears have higher contact ratios and, as a result, produce smoother output. They also tend to be quieter. One disadvantage of helical gears is that they produce axial gear loads that must be constrained to maintain drive stiffness.
The limiting factor in gear transmission stiffness is the stiffness of the gear teeth; each tooth acts as an elastic cantilever during the time that it is loaded. To maximize stiffness, the largest possible gear diameters should be chosen.
A common revolute joint transmission element in robot design is the harmonic drive. These drives feature in-line parallel shafts and very high transmission ratios in compact packages. With selective assembly procedures, near zero backlash harmonic drives can be produced. Static friction in these drives, and manufacturing tolerances often result in cyclic friction torque variation called cogging. Also, such drives are difficult to back-drive when the robot is taught by leading it through its motions manually.
Power is often transmitted in robots through torsion shafts or weight-saving torque tubes. Transmitting power at low torque and high angular velocities minimizes shaft diameter, wall thickness, and weight.
Several robot manufacturers use toothed positive drive belts as transmission elements. They are used primarily when low-cost power transmission is required over large distances, or as a simple interface between a drive motor and the first stage of gear reduction. Transmission ratios are limited because there is generally a minimum pulley size based on belt fatigue life. Drive stiffness in a belt transmission is a function of the belt material and belt tensioning system. While light in weight, backlash and vibration make the use of such belts problematic. The Beyer U.S. Pat. No. 4,608,884 discloses gear belts which extend between pulleys which, in turn, are attached to the shafts of a pair of reduction gears in a robot head.
Another important consideration in evaluating transmission system performance is efficiency. Most robot transmission elements have good efficiencies when they are transmitting at or near their rated power levels. However, transmissions with high static friction such as harmonic drives with a low backlash option or belt drives with heavily preloaded bearings and high belt pretension are not very efficient at power transmission levels that are a small percentage of their rated limits.
Other important considerations in choosing transmission elements are system geometry, compactness, and simplicity. Planetary spur gears and harmonic drives are among the most compact forms of transmission elements.
The Helms U.S. Pat. No. 4,499,790 discloses a swivel head for industrial robots having two degrees of freedom. The head has two intersecting axes and uses planetary gears.
One of the most pertinent prior art patents is the Pardo et al. U.S. Pat. No. 4,047,448 which discloses a robot hand or wrist having a stationary control casing, three perpendicular shafts and corresponding axes and members. The members are supported for driving rotation about one of the axes by servo motors in the control casing.
In choosing transmission elements, one must also consider the time required for adjustment and set-up procedures. Proper backlash control of gears requires adjustments of distance between gear centers.
Smoothness of the power transmission is another design consideration, especially where low-speed motion is required such as in arc welding. Bands and linkage provide a smooth drive. Ball screws also provide smooth drives if they are clean and in good condition. Gears are not as smooth as screws and belts, but, in general, they are smoother than chains and harmonic drives. Proper setup is critical for smoothness of operation of gears.
In general, when large reduction ratios are required, transmission elements of choice will be harmonic drives, gears, or ball screws. For a low transmission ratio, rack and pinion drives, single-stage gearing, belts, cables, or chains are used.